This invention relates to tufting machines and more particularly to apparatus for increasing the range of operation of a needle bar positioning device to jog the needles rapidly relatively to the base material.
In the production of tufted fabrics it is known to jog or shift the needle bar transversely across the tufting machine relatively to the base material in order to break up the unattractive alignment of the longitudinal rows of tufts known as the "corncob" effect and also to reduce the difficulty of visible optical streaking resulting from variations in coloration of the yarn, the latter difficulty being a result of variations in the dye absorbing property of heat set yarns. Various devices have been proposed and are in use for controllably applying a step-wise force to the needle bar of the tufting machine in accordance with a pattern. For example, the primary means for supplying this jogging has been a pattern cam driven in timed relationship to the reciprocation of the needle bar and acting upon a cam follower mechanism coupled to the needle bar. Exemplary of this prior art device are U.S. Pat. Nos. 3,026,830 and 3,934,524. Because of the limited patterning capabilities of a cam, and especially the limited longitudinal pattern repeat, the cam providing a longitudinal repeat every revolution of the cam, prior art needle bar shifters having patterning capabilities beyond that provided by the cam and follower systems have been developed. The known systems are disclosed in U.S. Pat. Nos. 3,964,408 and 3,972,295, which utilize pawl and ratchet devices, U.S. Pat. No. 4,010,700 which uses an indexing device, and U.S. Pat. No. 4,173,192 which uses a hydraulic actuator.
In these expanded prior art needle positioning devices the inherent inertia of the mechanical parts and of the hydraulic fluid, together with the inertia of the needle bar, is such that the system is substantially slower than that required by the tufting machine. Both the positioning apparatus and the needle bar are stepped laterally during the same portion of the tufting machine cycle. A substantial force from the positioner must be exerted during this part of the cycle to move the needle bar directly. Normally the needles are out of the base material fabric or the needle plate fingers for approximately 110.degree. of the needle bar reciprocation cycle and the lateral or transverse shift must occur during this portion of the cycle in order to avoid damage to the needles. Thus, since only this small portion of the overall cycle is available for applying the force to move the needle bar laterally and since the positioners require a relatively large amount of time to perform the shift, the tufting machine is slowed excessively to increase the time the needles are free of the backing. In most cases tufting machines operating with a shifting needle bar mechanism run at approximately one third to one half the speed the tufting machine would be run without the shifter and the tufting machine therefore produces one third to one half of the pile fabric in the same period of time that it could.